This specification includes an Appendix A, which includes a listing of a computer program included in the preferred embodiment described below. Appendix A is provided on a compact disc containing the file Code2.txt (16 Kilobytes, created Jun. 13, 2002), which file is hereby incorporated by reference. This appendix includes copyrighted subject matter, and the assignee of the present invention retains all copyright rights, other than the right to reproduce the appendix as a portion of the specification.
This invention relates to methods and systems for equalizing charge on a set of series-connected batteries as the batteries are being charged.
Many automotive electrical systems require a higher operating voltage than that available from the conventional 12.6 V battery. This is because automotive electrical power needs have increased significantly over the past decade, and it is more efficient to supply high power by using a higher voltage rather than by simply increasing the amperage. Operation of such systems requires diligent monitoring and control of the individual battery voltages in order to maintain equal charging of the batteries.
When a vehicle engine starts, the alternator produces a regulated voltage across the battery terminals for charging. Typically, an alternator output voltage of 14.0 V is used to charge a 12.6 V battery. Series-connected batteries generally provide 14.0 V charge increments for each battery in the series. For example, when six batteries are connected in series, the alternator is designed to produce an 84 V charging voltage across the entire string.
Under normal operating conditions, battery charge characteristics vary depending upon many factors, including the following:
1. Age of battery: older batteries accept more current then newer batteries;
2. State of charge: the charge current varies with time during the charging process;
3. Ambient temperature: batteries accept less charge as temperature drops;
4. Battery construction: details of battery design affect charging characteristics.
Variations between individual batteries within a single string will often cause overcharging or under charging in one or more of the batteries of the string. Any battery in the series-connected string that once attains an overcharged state tends to persist in that state during charging until battery failure occurs.
Chen U.S. Pat. No. 6,008,623 and Podrazhansky U.S. Pat. No. 5,504,415 both disclose charge equalizers for a series-connected battery string. The Chen system activates flyback converters to draw out the overcharging currents of the batteries in the string. Overcharging currents are detected by comparing the voltage across a battery or cell against a fixed reference. The Podrazhansky system measures the voltage across a battery to determine the state of charge of the battery. A circuit that selectively shunts charging current around the battery is provided, and, depending upon the state of charge of each battery, a portion of the charging current may be shunted around the battery to prevent overcharging.
The systems described above suffer from the disadvantage that the charging currents applied to individual batteries are only limited after the battery voltage has exceeded a preselected threshold voltage. For this reason, these systems do not equalize the rate of charging of individual batteries prior to the time any individual one of the batteries reaches the threshold voltage.
The charging systems described below provide a shunt path for each of a set of series-connected batteries. Each shunt path includes a respective switch and at least one respective load. On a repeating basis, all of the switches are placed in the open-circuit condition, and then the voltage across each of the batteries is measured. The largest measured voltage is determined, and then the switch of the shunt path associated with the largest measured voltage is closed for a predetermined time. At this point the cycle repeats, all of the switches are opened, and the battery voltages are again measured.
The disclosed system reduces charging current applied to the highest-voltage battery, whether or not the highest-voltage battery is in an over charged condition. The undercharged batteries therefore receive relatively more charging current than the batteries at a higher charge state. In this way, the state of charge of the individual batteries is maintained more nearly equal throughout the period that the batteries are being charged to a full state of charge.
This section has been provided by way of general introduction, and it should not be used to narrow the scope of the following claims.